Method of communications in a backscatter system, interrogator, and backscatter communications system

ABSTRACT

An interrogator for use in a backscatter system, the interrogator comprising an antenna configured to receive a backscatter signal; an IQ downconverter coupled to the antenna and configured to downconvert the backscatter signal to produce I and Q signals; a combiner coupled to the IQ downconverter and configured to combine the I and Q signals to produce a combined signal; and an analog to digital converter coupled to the combiner and configured to convert the combined signal to a digital signal. A method of communications in a backscatter system, the method comprising receiving a backscatter signal; downconverting the backscatter signal with an IQ downconverter to produce I and Q signals; combining the I and Q signals to produce a combined signal; and converting the combined signal to a digital signal.

CROSS REFERENCE TO RELATED APPLICATION

This is a Continuation of U.S. patent application Ser. No. 09/080,624,filed May 18, 1998, now U.S. Pat. No. 6,075,973, and titled “Method ofCommunicating in a Backscatter System, Interrogator, and BackscatterCommunications System”.

TECHNICAL FIELD

The invention relates to wireless communication systems. Moreparticularly, the invention relates to backscatter communicationsystems.

BACKGROUND OF THE INVENTION

Backscatter communication systems are known in the art. In a backscattersystem, one transponder, such as an interrogator, sends out a command toa remote communications device. After the interrogator transmits thecommand, and is expecting a response, the interrogator switches to a CWmode (continuous wave mode). In the continuous wave mode, theinterrogator does not transmit any information. Instead, theinterrogator just transmits radiation at a certain frequency. In otherwords, the signal transmitted by the interrogator is not modulated.After a remote communications device receives a command from theinterrogator, the remote communications device processes the command.The remote communications device of the backscatter system modulates thecontinuous wave by switching between absorbing RF radiation andreflecting RF radiation. For example, the remote communications devicealternately reflects or does not reflect the signal from theinterrogator to send its reply. Two halves of a dipole antenna can beeither shorted together or isolated from each other to modulate thecontinuous wave.

One example of a backscatter system is described in commonly assignedU.S. patent application Ser. No. 08/705,043, filed Aug. 29, 1996, andincorporated herein by reference. Another example of a backscattersystem is described in U.S. Pat. No. 5,649,296 to MacLellan et al. whichis also incorporated herein by reference.

In backscatter systems, the reflected backscatter signal can be returnedto an interrogator in any phase because the distance between the remotecommunications device and the interrogator is unknown. Phase is afunction of distance. Therefore, an IQ downconverter (e.g., a quadraturedownconverter) is included in the interrogator. In an IQ downconverter,the local signal is mixed with the reflected backscatter signal toproduce an in phase signal I. The local signal is mixed with thereflected backscatter signal, after either the local signal or thereflected signal is phase shifted 90 degrees, to produce a quadraturesignal Q. Depending on the phase of the reflected backscatter signal,when the reflected backscatter signal is mixed with the local signal theresult may be a positive voltage, a negative voltage, or no voltage atall. When a periodic signal reaches its peak, a 90 degree phase shiftedversion of the same signal reaches zero. By mixing at a 90 degrees phaseshift as well as mixing the reflected signal without a phase shift, asignal be found for certain somewhere on the I output or Q output, orboth. An IQ downconverter is described in U.S. Pat. No. 5,617,060 toWilson et al., which is incorporated herein by reference.

Circuitry is typically coupled to each of the I and Q signals forvarious processing steps before the resultant signals are combined intoone channel. This can involve duplication of circuitry.

One application for backscatter communications is in wireless electronicidentification systems, such as those including radio frequencyidentification devices. Of course, other applications for backscattercommunications exist as well. Most presently available radio frequencyidentification devices utilize a magnetic coupling system. Anidentification device is usually provided with a unique identificationcode in order to distinguish between a number of different devices.Typically, the devices are entirely passive (have no power supply),which results in a small and portable package. However, suchidentification systems are only capable of operation over a relativelyshort range, limited by the size of a magnetic field used to supplypower to the devices and to communicate with the devices.

Another wireless electronic identification system utilizes a large,board level, active transponder device affixed to an object to bemonitored which receives a signal from an interrogator. The devicereceives the signal, then generates and transmits a responsive signal.The interrogation signal and the responsive signal are typicallyradio-frequency (RF) signals produced by an RF transmitter circuit.Because active devices have their own power sources, and do not need tobe in close proximity to an interrogator or reader to receive power viamagnetic coupling. Therefore, active transponder devices tend to be moresuitable for applications requiring tracking of something that may notbe in close proximity to an interrogator, such as a railway car.

SUMMARY OF THE INVENTION

The invention provides an interrogator for use in a backscatter system.The interrogator includes an antenna configured to receive a backscattersignal. An IQ downconverter is coupled to the antenna and configured todownconvert the backscatter signal to produce I and Q signals. Acombiner is coupled to the IQ downconverter and configured to combinethe I and Q signals to produce a combined signal. An analog to digitalconverter is coupled to the combiner and configured to convert thecombined signal to a digital signal.

In one aspect of the invention, the combiner is an analog combiner. Inanother aspect of the invention, the combiner is coupled directly to theIQ downconverter.

Another aspect of the invention provides a system including a wirelesscommunications device. The wireless communications device includes anintegrated circuit having a processor. The integrated circuit furtherincludes a memory, a receiver, and a backscatter modulator coupled tothe processor. The system further includes an interrogator configured totransmit a command to the wireless communications device andsubsequently transmit a continuous wave to the wireless communicationsdevice for modulation by the backscatter modulator. The interrogatorincludes an IQ downconverter configured to receive a modulatedbackscatter signal from the wireless communications device and toproduce I and Q signals. The interrogator further includes a combinercoupled to the downconverter and configured to produce a combinedsignal.

In one aspect of the invention, the combiner is coupled directly to thedownconverter.

Another aspect of the invention provides a method of communications in abackscatter system. The method comprises receiving a backscatter signal.The backscatter signal is downconverted with an IQ downconverter toproduce I and Q signals. The I and Q signals are combined to produce acombined signal. The combined signal is converted to a digital signal.

Circuitry coupled to the I signal was duplicated for the Q signal inprior designs. By combining the I and Q signals earlier than in otherdesigns, duplication of circuitry is reduced.

BRIEF DESCRIPTION OF THE DRAWINGS

Preferred embodiments of the invention are described below withreference to the following accompanying drawings.

FIG. 1 is a block diagram illustrating a communication system embodyingthe invention.

FIG. 2 is a front view of an employee badge according to one embodimentthe invention.

FIG. 3 is a front view of a radio frequency identification tag accordingto another embodiment of the invention.

FIG. 4 is a circuit schematic of a transponder included in the system ofFIG. 1.

FIG. 5 is a block diagram of an interrogator in accordance with oneembodiment of the invention.

FIG. 6 is a circuit schematic of DPSK circuitry included in theinterrogator of FIG. 5.

FIG. 7 is a circuit schematic of RF circuitry included in theinterrogator of FIG. 5.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

This disclosure of the invention is submitted in furtherance of theconstitutional purposes of the U.S. Patent Laws “to promote the progressof science and useful arts” (Article 1, Section 8).

FIG. 1 illustrates a wireless communications system 10 embodying theinvention. The communications system 10 includes a first transponderincluding an interrogator 26 and a host computer 48 in communicationwith the interrogator 26. The communications system 10 further includesa wireless communications device 12, such as the device disclosed inU.S. patent application Ser. No. 08/705,043, filed Aug. 29, 1996. In oneembodiment, the wireless communications device 12 comprises a wirelessidentification device such as the Microstamp (TM) integrated circuitavailable from Micron Communications, Inc., 3176 S. Denver Way, Boise,Id. 83705. The interrogator 26 communicates with the communicationsdevice 12 via an electromagnetic link, such as via an RF link (e.g., atmicrowave frequencies, in one embodiment). While other embodiments arepossible, in the illustrated embodiment, the communications device 12includes a transponder 16 having a receiver 30 and a transmitter 32. Thecommunications device 12 further includes a power source 18 connected tothe transponder 16 to supply power to the transponder 16. Thecommunications device 12 further includes at least one antenna connectedto the transponder 16 for wireless transmission and reception. In theillustrated embodiment, the communications device 12 includes at leastone antenna 46 connected to the transponder 16 for radio frequencytransmission by the transponder 16, and at least one receive antenna 44connected to the transponder 16 for radio frequency reception by thetransponder 16. In the illustrated embodiment, the transmit antenna 46is a dipole antenna, and the receive antenna 44 is a loop antenna. Inthe illustrated embodiment, the transponder 16 is in the form of anintegrated circuit. However, in alternative embodiments, all of thecircuitry of the transponder 16 is not necessarily all included in asingle integrated circuit.

The power source 18 is a thin film battery in the illustratedembodiment; however, in alternative embodiments, other forms of powersources can be employed. If the power source 18 is a battery, thebattery can take any suitable form. Preferably, the battery type will beselected depending on weight, size, and life requirements for aparticular application. In one embodiment, the battery 18 is a thinprofile button-type cell forming a small, thin energy cell more commonlyutilized in watches and small electronic devices requiring a thinprofile. A conventional button-type cell has a pair of electrodes, ananode formed by one face and a cathode formed by an opposite face. In analternative embodiment, the battery comprises a series connected pair ofbutton type cells.

The communications device 12 can be included in any appropriate housingor packaging.

FIG. 2 shows but one example of a housing in the form of a card 11comprising plastic or other suitable material. The plastic card 11houses the communications device 12 to define an employee identificationbadge 13 including the communications device 12. In one embodiment, thefront face of the badge 13 has visual identification features such as anemployee photograph or a fingerprint in addition to identifying text.

FIG. 3 illustrates but one alternative housing supporting the device 12.More particularly, FIG. 3 illustrates a miniature housing 20 encasingthe device 12 to define a tag which can be supported by an object (e.g.,hung from an object, affixed to an object, etc.).

Although two particular types of housings have been disclosed, thecommunications device 12 can be included in any appropriate housing. Thecommunications device 12 is of a small size that lends itself toapplications employing small housings, such as cards, miniature tags,etc. Larger housings can also be employed. The communications device 12,housed in any appropriate housing, can be supported from or attached toan object in any desired manner.

The interrogator unit 26 includes a plurality of antennas, as well astransmitting and receiving circuitry, similar to that implemented in thedevice 16. The host computer 48 acts as a master in a master-slaverelationship with the interrogator 26. The host computer 48 includes anapplications program for controlling the interrogator 26 andinterpreting responses, and a library of radio frequency identificationdevice applications or functions. Most of the functions communicate withthe interrogator 26. These functions effect radio frequencycommunication between the interrogator 26 and the communications device12.

The communications system 10 includes a transmit antenna X1, and areceive antenna R1 connected to the interrogator 26. In operation, theinterrogator 26 transmits an interrogation signal or command 27(“forward link”) via the antenna X1. The communications device 12receives the incoming interrogation signal via its antenna 44. Uponreceiving the signal 27, the communications device 12 responds bygenerating and transmitting a responsive signal or reply 29 (“returnlink”). The interrogator 26 is described in greater detail below.

In one embodiment, the responsive signal 29 is encoded with informationthat uniquely identifies, or labels the particular device 12 that istransmitting, so as to identify any object or person with which thecommunications device 12 is associated.

In the embodiment illustrated in FIG. 1, multiple communications devices12 can be employed; however, there is no communication between multipledevices 12. Instead, the multiple communications devices 12 communicatewith the interrogator 26. FIG. 1 illustrates the communications device12 as being in the housing 20 of FIG. 3. The system would operate in asimilar manner if the device 12 is provided in a housing such as thehousing 10 of FIG. 2, or any other appropriate housing. Multiplecommunications devices 12 can be used in the same field of aninterrogator 26 (i.e., within communications range of an interrogator26). Similarly, multiple interrogators 26 can be in proximity to one ormore of the devices 12.

The above described system 10 is advantageous over prior art devicesthat utilize magnetic field effect systems because, with the system 10,a greater range can be achieved, and more information can be obtained(instead of just an identification number).

As a result, such a system 10 can be used, for example, to monitor largewarehouse inventories having many unique products needing individualdiscrimination to determine the presence of particular items within alarge lot of tagged products.

FIG. 4 is a high level circuit schematic of the transponder 16 utilizedin the devices of FIGS. 1-3. In the embodiment shown in FIG. 4, thetransponder 16 is a monolithic integrated circuit. More particularly, inthe illustrated embodiment, the integrated circuit 16 if comprises asingle die, having a size of 209×116 mils², including the receiver 30,the transmitter 32, a micro controller or microprocessor 34, a wake uptimer and logic circuit 36, a clock recovery and data recovery circuit38, and a bias voltage and current generator 42.

In one embodiment, the communications devices 12 switch between a“sleep” mode of operation, and higher power modes to conserve energy andextend battery life during periods of time where no interrogation signal27 is received by the device 12, using the wake up timer and logiccircuitry 36.

In one embodiment, a spread spectrum processing circuit 40 is includedin the transponder 16. In this embodiment, signals transmitted andreceived by the interrogator 26, and transmitted and received by thecommunications device 12 are modulated spread spectrum signals. Otherembodiments are possible.

Many modulation techniques minimize required transmission bandwidth.However, the spread spectrum modulation technique employed in theillustrated embodiment requires a transmission bandwidth that is up toseveral orders of magnitude greater than the minimum required signalbandwidth. Although spread spectrum modulation techniques are bandwidthinefficient in single user applications, they are advantageous wherethere are multiple users, as is the case with the instant radiofrequency identification system 24. The spread spectrum modulationtechnique of the illustrated embodiment is advantageous because theinterrogator signal can be distinguished from other signals (e.g.,radar, microwave ovens, etc.) operating at the same frequency. Thespread spectrum signals transmitted by the communications device 12 andby the interrogator 26 are pseudo random and have noise-like propertieswhen compared with the digital command or reply. The spreading waveformis controlled by a pseudo-noise or pseudo random number (PN) sequence orcode. The PN code is a binary sequence that appears random but can bereproduced in a predetermined manner by the device 12. Moreparticularly, incoming spread spectrum signals are demodulated by thecommunications device 12 or by the interrogator 26 through crosscorrelation with a version of the pseudo random carrier that isgenerated by the communications device 12 itself or the interrogator 26itself, respectfully. Cross correlation with the correct PN sequenceunspreads the spread spectrum signal and restores the modulated messagein the same narrow band as the original data.

A pseudo-noise or pseudo random sequence (PN sequence) is a binarysequence with an autocorrelation that resembles, over a period, theautocorrelation of a random binary sequence. The autocorrelation of apseudo-noise sequence also roughly resembles the autocorrelation ofband-limited white noise. A pseudo-noise sequence has manycharacteristics that are similar to those of random binary sequences. isFor example, a pseudo-noise sequence has a nearly equal number of zerosand ones, very low correlation between shifted versions of the sequence,and very low cross correlation between any two sequences. A pseudo-noisesequence is usually generated using sequential logic circuits. Forexample, a pseudo-noise sequence can be generated using a feedback shiftregister.

A feedback shift register comprises consecutive stages of two statememory devices, and feedback logic. Binary sequences are shifted throughthe shift registers in response to clock pulses, and the output of thevarious stages are logically combined and fed back as the input to thefirst stage. The initial contents of the memory stages and the feedbacklogic circuit determine the successive contents of the memory.

The illustrated embodiment employs direct sequence spread spectrummodulation. A direct sequence spread spectrum (DSSS) system spreads thebaseband data by directly multiplying the baseband data pulses with apseudo-noise sequence that is produced by a pseudo-noise generator. Asingle pulse or symbol of the PN waveform is called a “chip.”Synchronized data symbols, which may be information bits or binarychannel code symbols, are added in modulo-2 fashion to the chips beforebeing modulated. The receiver performs demodulation. For example, in oneembodiment the data is amplitude modulated. Assuming that codesynchronization has been achieved at the receiver, the received signalpasses through a wideband filter and is multiplied by a local replica ofthe PN code sequence. This multiplication yields the unspread signal.

A pseudo-noise sequence is usually an odd number of chips long. In theillustrated embodiment, one bit of data is represented by a thirty-onechip sequence. A zero bit of data is represented by inverting thepseudo-noise sequence.

Spread spectrum techniques are also disclosed in “Spread SpectrumSystems,” by R. C. Dixon, published by John Wiley and Sons, Inc.

In operation, the interrogator sends out a command that is spread arounda certain center frequency (e.g, 2.44 GHz). After the interrogatortransmits the command, and is expecting a response, the interrogatorswitches to a CW mode (continuous wave mode). In the continuous wavemode, the interrogator does not transmit any information. Instead, theinterrogator just transmits 2.44 GHz radiation. In other words, thesignal transmitted by the interrogator is not modulated. After thecommunications device 12 receives the command from the interrogator, thecommunications device 12 processes the command. If the communicationsdevice 12 is in a backscatter mode it alternately reflects or does notreflect the signal from the interrogator to send its reply. For example,in the illustrated embodiment, two halves of a dipole antenna are eithershorted together or isolated from each other to send a reply.

When the interrogator sends a command to a communications device 12, itcan be important whether the command was performed by the communicationsdevice 12 and whether the interrogator knows that the command wasperformed. Therefore, it may be desirable to have more margin on thereturn link than on the forward link so that if the interrogator sends acommand to a communications device 12, the interrogator can bereasonably sure to get a reply. Otherwise, if the communications device12 is on the fringe of the communications range, it may have received acommand from the interrogator to change its mode of operation withoutthe interrogator being able to receive an acknowledgment back from thecommunications device 12. There may also be scenarios where it isdesirable to have more margin on the forward link than the return link.Thus, in the illustrated embodiment, the power level for the forwardlink can be set independently from the power level for the return link,as will be described below in greater detail.

In one embodiment, the clock for the entire integrated circuit 16 isextracted from the incoming message itself by clock recovery and datarecovery circuitry 38. This clock is recovered from the incomingmessage, and used for timing for the micro controller 34 and all theother clock circuitry on the chip, and also for deriving the transmittercarrier or the subcarrier, depending on whether the transmitter isoperating in active mode or backscatter mode.

In addition to recovering a clock, the clock recovery and data recoverycircuit 38 also performs data recovery on valid incoming signals. Thevalid spread spectrum incoming signal is passed through the spreadspectrum processing circuit 40, and the spread spectrum processingcircuit 40 extracts the actual ones and zeros of data from the incomingsignal. More particularly, the spread spectrum processing circuit 40takes the chips from the spread spectrum signal, and reduces eachthirty-one chip section down to a bit of one or zero, which is passed tothe micro controller 34.

The micro controller 34 includes a serial processor, or I/O facilitythat received the bits from the spread spectrum processing circuit 40.The micro controller 34 performs further error correction. Moreparticularly, a modified hamming code is employed, where each eight bitsof data is accompanied by five check bits used by the micro controller34 for error correction. The micro controller 34 further includes amemory, and after performing the data correction, the micro controller34 stores bytes of the data bits in memory. These bytes contain acommand sent by the interrogator 26. The micro controller 34 responds tothe command.

For example, the interrogator 26 may send a command requesting that anycommunications device 12 in the field respond with the device'sidentification number. Status information is also returned to theinterrogator 26 from the communications device 12 when thecommunications device 12 responds.

The transmitted replies have a format similar to the format of incomingmessages. More particularly, a reply starts with a preamble (e.g., allzeros in active mode, or alternating double zeros and double ones inbackscatter mode), followed by a Barker or start code, followed byactual data.

The incoming message and outgoing reply preferably also include a checksum or redundancy code so that the integrated circuit 16 or theinterrogator 12 can confirm receipt of the entire message or reply.

The interrogator 26 provides a communication link between a hostcomputer and the transponder 16. The interrogator 26 connects to thehost computer 48 via an IEEE-1284 enhanced parallel port (EPP). Theinterrogator communicates with the transponder 16 via the RF antennasX1, and R1.

A Maximal Length Pseudo Noise (PN) Sequence is used in the DirectSequence Spread Spectrum (DSSS) communications scheme in the forwardlink. In one embodiment, the sequence is generated by a linear feedbackshift register. This produces a repeating multiple “chip” sequence.

A zero bit is transmitted as one inverted full cycle of the PN sequence.A one bit is transmitted as one full non-inverted cycle of the PNsequence.

After sending a command, the interrogator sends a continuous unmodulatedRF signal with a frequency of 2.44175 GHz. Return link data isDifferential Phase Shift Key (DPSK) modulated onto a square wavesubcarrier with a frequency of 596.1 KHz. A data 0 corresponds to onephase and data 1 corresponds to another, shifted 180 degrees from thefirst phase. The subcarrier is used to modulate antenna impedance of atransponder 16. For a simple dipole, a switch between the two halves ofthe dipole antenna is opened and closed. When the switch is closed, theantenna becomes the electrical equivalent of a single half-wavelengthantenna that reflects a portion of the power being transmitted by theinterrogator. When the switch is open, the antenna becomes theelectrical equivalent of two quarter-wavelength antennas that reflectvery little of the power transmitted by the interrogator. In oneembodiment, the dipole antenna is a printed microstrip half wavelengthdipole antenna.

In one embodiment (see FIG. 5), the interrogator 26 includes enhancedparallel port (EPP) circuitry 50, DPSK (differential phase shift keyed)circuitry 52, and RF (radio frequency) circuitry 54, as well as a powersupply (not shown) and a housing or chassis (not shown). In theillustrated embodiment, the enhanced parallel port circuitry 50, theDPSK circuitry 52, and the RF circuitry 54 respectively define circuitcard assemblies (CCAs). The interrogator uses an IEEE-1284 compatibleport in EPP mode to communicate with the host computer 48. The EPPcircuitry 50 provides digital logic required to coordinate sending andreceiving a message with a transponder 16. The EPP circuitry 50 buffersdata to transmit from the host computer 48, converts the data to serialdata, and encodes it. The EPP circuitry 50 then waits for data from thetransponder 16, converts it to parallel, and transfers it to the hostcomputer 48. In one embodiment, messages include up to 64 bytes of data.

The EPP mode interface provides an asynchronous, interlocked, byte wide,bidirectional channel controlled by a host device. The EPP mode allowsthe host computer to transfer, at high speed, a data byte to/from theinterrogator within a single host computer CPU I/O cycle (typically 0.5microseconds per byte).

The DPSK circuitry 52 (see FIG. 6) receives signals I and Q from the RFcircuitry 54 (described below), which signals contain the DPSK modulatedsub-carrier. The DPSK circuitry 52 includes a combiner 64, coupled tothe I and Q signals, combining the analog signals to produce a combinedsignal 85. The combiner 64 includes a phase shifter 87 configured toshift one of the signals I and Q by 90 degrees before the signals arecombined in an adder 89. The combiner 64 is an analog quadraturecombiner. In the illustrated embodiment, the combiner 64 is coupleddirectly to a quadrature downconverter 84 (FIG. 7).

The DPSK circuitry 52 further includes an amplifier 86 coupled to thecombiner 64 and configured to amplify the combined signal 85 to producean amplified signal 90. The DPSK circuitry 52 further includes automaticgain control circuitry 88 coupled to the amplifier 86 and configured toamplify the amplified signal 90 to produce a twice amplified signal 91.In the illustrated embodiment, the automatic gain control circuitry 88comprises a voltage controlled amplifier.

The DPSK circuitry 52 further includes an analog to digital converter 72coupled to the automatic gain control circuitry 88 and configured toconvert the combined signal to a digital signal.

The DPSK circuitry 52 further includes an analog to digital converter 72coupled to the automatic gain control circuitry 88 to convert the signal91 from an analog signal to a digital signal. The DPSK circuitry 52further includes a bit synchronizer 74 coupled to the filter 72 forregeneration of the data clock. The DPSK circuitry 52 further includeslock detect circuitry 76 coupled to the low pass filter 72 andgenerating a lock detect signal. The data, clock, and lock detect signalare sent to the EPP circuitry 50.

The RF circuitry 54 (see FIG. 7) interfaces with the transmit andreceive antennas X1, and R1. The RF circuitry modulates the data fortransmission to a transponder 16, provides a continuous wave (CW)carrier for backscatter communications with a transponder 16 (ifbackscatter communications are employed), and receives and downconvertsthe signal received from the transponder unit (which is a backscattersignal in one embodiment).

The RF circuitry 54 also includes a power divider 73, and a frequencysynthesizer 75 coupled to the power divider 73. The frequencysynthesizer 75 tunes the RF continuous waver carrier. The RF circuitrydefines a transmitter, and receives data from the EPP circuitry 50. TheRF circuitry 54 includes an amplitude modulation (AM) switch 77 thatreceives the data from the EPP circuitry 50 and amplitude modulates thedata onto a carrier. More particularly, the AM switch 77 turns the RF onand off (ON OFF KEY). The RF circuitry 54 further includes a poweramplifier 79, coupled to the AM switch 77, which amplifies the signal.

During continuous wave (CW) transmission for the backscatter mode, theAM switch 74 is left in a closed position. When the interrogator 26 istransmitting in the CW mode, the transponder 16 backscatters the signalwith a DPSK modulated sub carrier. This signal is received via thereceive antenna R1. The RF circuitry 54 further includes a low noiseamplifier (LNA) 82 coupled to the antenna R1 and amplifying the receivedsignal. The RF circuitry 54 further includes a quadrature downconverter84, coupled to the LNA 82, coherently downconverting the receivedsignal. The quadrature downconverter mixes the received signal with alocally generated signal from the frequency synthesizer 75 and a 90degree phase shifted signal to produce baseband signals I and Q(in-phase and quadrature signals). The I and Q signals, which containthe DPSK modulated sub-carrier, are passed on to the DPSK circuitry 52(FIG. 6) for demodulation.

An example of a command that can be sent from the interrogator 26 to thecommunications device 12 is an Identify command.

An Identify command is used when attempting to determine theidentification of one or more of the devices 12. Each communicationsdevice 12 has its own identification number “TagId.” It is possible thatthe interrogator will receive a garbled reply if more than one tagresponds with a reply. If replies from multiple tags are received, anarbitration scheme is used to isolate a single communications device 12.

A WriteDigitalPort command is used to write data to the synchronousserial port of a communications device 12.

A WriteTagId command is used to update the Tagld of a communicationsdevice 12.

An IdentifyAll command returns the number of communications devices 12found within the system's communication range.

Thus, an interrogator has been provided including a combined coupled toa quadrature downconverter. By combining the I and Q signals in aninterrogator earlier than in other designs, duplication of circuitry isreduced.

In compliance with the statute, the invention has been described inlanguage more or less specific as to structural and methodical features.It is to be understood, however, that the invention is not limited tothe specific features shown and described, since the means hereindisclosed comprise preferred forms of putting the invention into effect.The invention is, therefore, claimed in any of its forms ormodifications within the proper scope of the appended claimsappropriately interpreted in accordance with the doctrine ofequivalents.

What is claimed is:
 1. A backscatter receiver comprising: an IQdownconverter configured to convert a received backscatter signal toproduce I and Q signals; a combiner coupled to the IQ downconverter andconfigured to combine the I and Q signals to produce a combined signal;and an analog to digital converter coupled to the combiner andconfigured to convert the combined signal to a digital signal.
 2. Abackscatter receiver in accordance with claim 1 and further comprisingautomatic gain control circuitry coupled between the combiner and theanalog to digital converter.
 3. A backscatter receiver in accordancewith claim 1 wherein the combiner is a quadrature combiner.
 4. Abackscatter receiver in accordance with claim 1 and further comprisingan amplifier coupled between the combiner and the analog to digitalconverter.
 5. A backscatter receiver in accordance with claim 1 andfurther comprising an amplifier coupled to the combiner and configuredto amplify the combined signal to produce an amplified signal, and avoltage controlled amplifier coupled to the first mentioned amplifierand configured to further amplify the amplified signal.
 6. A backscatterreceiver comprising: an antenna; an IQ downconverter coupled to theantenna and configured to downconvert a backscatter signal received bythe antenna to produce I is and Q signals; a combiner coupled to the IQdownconverter and configured to combine the I and Q signals to produce acombined signal; and an analog to digital converter coupled to thecombiner and configured to convert the combined signal to a digitalsignal.
 7. A backscatter receiver in accordance with claim 6 wherein thecombiner is a quadrature combiner.
 8. A backscatter receiver inaccordance with claim 6 wherein the combiner is an analog combiner.
 9. Abackscatter receiver in accordance with claim 6 and further comprisingan amplifier coupled between the combiner and the analog to digitalconverter, the amplifier being configured to amplify the combinedsignal.
 10. A backscatter receiver comprising: an antenna; a quadraturedownconverter coupled to the antenna and configured to convert abackscatter signal received by the antenna to produce I and Q signals; acombiner coupled directly to the downconverter and configured to combinethe I and Q signals to produce a combined signal; and automatic gaincontrol circuitry coupled to the combiner and configured to amplify thecombined signal.
 11. A backscatter receiver in accordance with claim 10wherein the combiner is an analog quadrature combiner.
 12. A backscatterreceiver in accordance with claim 10 and further comprising an amplifiercoupled to the combiner and configured to amplify the combined signal.13. A backscatter receiver comprising: an antenna; a quadraturedownconverter coupled to the antenna and configured to convert abackscatter signal received by the antenna to produce I and Q signals; acombiner coupled directly to the downconverter and configured to combinethe I and Q signals to produce a combined signal; and an amplifiercoupled to the combiner and configured to amplify the combined signal toproduce an amplified signal, and a voltage controlled amplifier coupledto the first mentioned amplifier and configured to further amplify theamplified signal.
 14. A system comprising: a wireless communicationsdevice including an integrated circuit having a processor, and a memory,a receiver, and a backscatter modulator coupled to the processor; and atransponder including a transmitter configured to send a transmission tothe wireless communications device and subsequently transmit acontinuous wave to the wireless communications device for modulation bythe backscatter modulator, the transponder further including a receiverincluding an IQ downconverter configured to downconvert a modulatedbackscatter signal received from the wireless communications device intoI and Q signals, a combiner coupled to the IQ downconverter andconfigured to combine the I and Q signals to produce a combined signal,and an analog to digital converter coupled to the combiner andconfigured to convert the combined signal to a digital signal.
 15. Asystem in accordance with claim 14 wherein the transmitter is configuredto send the transmission at a microwave frequency.
 16. A system inaccordance with claim 15 wherein the transmitter is configured totransmit the continuous wave at a microwave frequency.
 17. A system inaccordance with claim 14 wherein the wireless communications deviceincludes a self contained power source coupled to the integratedcircuit.
 18. A system in accordance with claim 14 wherein the wirelesscommunications device includes a battery coupled to the integratedcircuit.
 19. A system comprising: a wireless communications deviceincluding an integrated circuit having a processor, and a memory, areceiver, and a backscatter modulator coupled to the processor; and atransponder including a transmitter configured to send a transmission tothe wireless communications device and subsequently transmit acontinuous wave to the wireless communications device for modulation bythe backscatter modulator, the transponder further including a receiverincluding an IQ downconverter configured to receive a modulatedbackscatter signal received from the wireless communications device andto produce I and Q signals, the transponder further including a combinercoupled directly to the IQ downconverter and configured to produce acombined signal, and the interrogator further including an analog todigital converter coupled to the combiner and configured to convert thecombined signal to a digital signal.
 20. A system in accordance withclaim 19 wherein the transmitter is configured to transmit an amplitudemodulated signal.